2′,3′-didehydro-3′-deoxythymidine (Stavudine) has the formula given below.

Stavudine is also known as d4T which is approved by U.S. FDA for the therapeutic treatment of patients infected with retroviruses.
Synthesis of Stavudine is first reported by J. P. Horwitz et al (J. Org. Chem. (1996) 31, 205) starting from 3′4′-dimesylthymidine as shown in Scheme-1.

In the above synthesis, 3′,5′-di-O-mesylthymidine is first treated with sodium hydroxide in refluxing water for 2 hours and the resultant 3,4-anhydrothymidine is treated with potassium t-butoxide in dimethyl sulfoxide (DMSO) at room temperature for 2 hrs. The reaction mixture is neutralized, evaporated to dryness and after a series of manipulations which included extraction, declourization, precipitation and recrystallization, gave Stavudine in 79% yield and an overall yield of 56%.
The above procedure is modified by Mansuri et al (J. Med. Chem. (1989) 32, 461) where the potassium t-butoxide/DMSO mixture is poured in to 30 volumes of toluene whereby the Stavudine precipitates as the potassium salt along with excess potassium tert-butoxide. This modification avoided the distillation of DMSO which caused decomposition of Stavudine. The obtained salt is neutralized in water and the precipitated product is extracted with acetone and then evaporated to dryness to give an off-white solid in 57% yield.
Adachi et al. [Carbohydrate Research (1979) 113] over-came some of the decomposition problems by employing sodium hydroxide in hexamethyl phosphorictriamide (HMPA). HMPA is removed by forming a chloroform complex in an aqueous mixture and Stavudine is isolated from the aqueous phase.
U.S. Pat. No. 5,539,099 discloses a modified route for producing Stavudine as shown in Scheme-2.

However, all the above discussed processes have one major draw back in common in that the final purity of Stavudine is less than the desired 99.5%. Known impurities which are difficult by using conventional techniques are still present, some of which were thymine, thymidine, threo-thymidine, 3,5-anhydrothymidine and 5′-O-[stavudin-5″-yl]-thero-thymidine being particularly difficult to remove.
Further U.S. Pat. No. 5,539,099 teaches the use of only N-methyl-2-pyrrolidinone as suitable to form a solvate of Stavudine whose recovery requires the use of an aprotic ester, amide or ketone solvent. No consideration is given for their use to form a solvate.
U.S. Pat. No. 6,635,753 discloses the novel Stavudine solvates of N,N-dimethyl acetamide, N,N-dimethylacrylamide, N,N-dimethylpropionamide etc. and recovering substantially pure Stavudine by breaking the solvate produced.
It is therefore an object of this invention to provide an improved process which is more efficient, using less toxic materials (for example, using solvents which are less toxic than N-Methyl-2-pyrrolidinone, DMA, DMAC and DMP) and providing pure product before recrystallization steps are carried out.